1. Field of the Invention
The present invention relates to a video camera, and more particularly to a video camera having a negative-positive reversal function.
2. Related Background Art
FIG. 1 is a block diagram showing the outline of the structure of a conventional television camera of this type having an iris control function. In FIG. 1, reference numeral 1 represents light from a subject (not shown). This light 1 passes through a lens 2 and iris 3 and becomes incident on an image pickup unit 4 whereat the light is photoelectrically converted into an image signal Y and outputted therefrom. The image signal Y is divided into a negative signal (Y) and a positive signal (Y) at a negative-positive inverter 5 which are then sent to a negative-positive switch unit 6. Reference numeral 5a represents an inverter amplifier. The negative-positive switch unit has a switch 6a which is switched under control of a negative-positive control signal (NP Cont). One of the negative and positive signals selected by the switch 6a is sent to a signal processing unit 7 and outputted therefrom as a video (Video) signal after having been processed.
In the meantime, a fraction of the image signal Y outputted from the image pickup unit 4 is sent to an iris control unit 8 whereat the image signal Y is adjusted to have a proper level and outputted to an iris driving unit 9. The iris driving unit 9 drives the iris 3 to control the light amount incident upon the image pickup unit 4. These units constitute an automatic iris control system.
The iris control unit 8 is constructed such that the image signal Y is controlled to have a proper level in accordance with an iris control signal (Iris Cont) supplied from an iris control signal generating circuit 17. The iris control signal Iris Cont can be adjusted externally by an operator by manipulating a manual volume 18, to thereby obtain a desired opening degree of the iris 3.
The conventional television camera described above, however, has a problem of poor handling in operation that the direction (polarity) of brightness change during iris adjustment is reversed between a mode of outputting a negative signal (hereinafter called a negative mode) and a mode of outputting a positive signal (hereinafter called a positive mode), resulting in a different way of adjustment between the negative mode and positive mode.
Specifically, in manipulating the manual volume 18 to supply an iris control signal and externally adjust the opening degree of the iris, the direction of brightness change of a video output signal is reversed between a positive mode image pickup and a negative mode image pickup. For example, in the positive mode, the iris is opened to make an image bright, whereas in the negative mode an image becomes dark if the iris is opened.
The reason for this will be described briefly. As the iris is opened in the positive mode, it is apparent that the image signal Y becomes stronger and the image becomes brighter. In the negative mode, the inverted negative signal Y is given by the following equation. EQU Y=A-Y (1)
where A is a constant. Therefore, as the iris is opened in the negative mode to make an image bright, the positive signal Y becomes stronger and so the negative signal Y becomes weaker, as understood from the equation (1). Thus, the relation between the direction of opening the iris and the image brightness is reversed between the negative and positive modes.
FIG. 2 is a block diagram showing the circuit arrangement of a conventional television camera of the type described above, having a white balance control function. In FIG. 2, reference numeral 21 represents an image pickup unit for photoelectrically converting light from a subject (not shown) into color signals E.sub.R ', E.sub.G ', and E.sub.B ' respectively of red (R), green (G), and blue (B) colors. Reference numeral 22 represents an amplifier unit having amplifiers 22a, 22b, and 22c for amplifying the color signals E.sub.R ', E.sub.G ', and E.sub.B '. The amplifiers 22a and 22c are voltage-controlled variable gain amplifiers, whose gains are controlled by control voltages V.sub.R and V.sub.B or white balance control information supplied from a white balance control signal generating circuit 38. Reference numeral 23 represents a signal processing unit which processes (gamma correction and the like) the amplifified color signals E.sub.R, E.sub.G, and E.sub.B and outputs image signals including a luminance signal E.sub.Y and color difference signals E.sub.R-Y and E.sub.B-Y. Reference numeral 24 represents an inverter unit having inverters 24a, 24b, and 24c for inverting the image signals. Reference numeral 25 represents a negative-positive switching unit for selectively using the inverted image signal (negative signal) and non-inverted image signal (positive signal) between the negative and positive modes, the negative-positive switching unit having switches 25a, 25b, and 25c which are controlled by a negative-positive control signal (NP CONT) supplied from a negative-positive switching signal generating circuit 36. Reference numeral 26 represents an encoder for converting the image signals into a video signal (Video) and outputting therefrom. Reference numeral 37 represents a manual volume for adjusting white balance control signals V.sub.R and V.sub.B.
The operation of the color television camera constructed as above will be described. Light from a subject is photoelectrically converted into R, G, and B color signals E.sub.R ', E.sub.G ', and E.sub.B ', which are amplified by the amplifiers 22a, 22b, and 22c and sent to the signal processing unit 23. The amplifiers 22a and 22c are voltage-controlled variable gain amplifiers, as described previously, and the gains thereof are controlled by the control voltages V.sub.R and V.sub.B to obtain a proper white balance. The signal processing unit 23 processes the color signals E.sub.R, E.sub.G, and E.sub.B and converts them into a luminance signal E.sub.Y and color difference signals E.sub.R-Y and E.sub.B-Y. These image signals outputted from the signal processing unit 23 pass through the inverter unit 24 and negative-positive switching unit 25 and are sent to the encoder 26 whereby they are converted into a video signal to be outputted therefrom.
The inverter unit 24 has inverters 24a, 24b, and 24c for respective signals E.sub.Y, E.sub.R-Y, and E.sub.B-Y, the interconnection between the switches 25a, 25b, and 25c of the negative-positive switching unit 25 being as shown in FIG. 2. As described above, the negative-positive switching unit 25 is controlled by the negative-positive control signal (NP CONT) supplied from the negative-positive switching signal generating circuit 36, to thereby activate the switches 25a, 25b, and 25c and selectively use the negative and positive signals. The inverters 24b and 24c for the color difference signals operate in the manner defined by the following equations. ##EQU1##
The inverter 24a for the luminance signal operates in the manner defined by the following equation. EQU E.sub.Y =A-E.sub.Y (II)
where A is a constant.
The operation of the white balance control by the circuit shown in FIG. 2 will be briefly described for the positive mode and for the negative mode.
(A) The case will be described wherein in the positive mode, the white balance control voltages V.sub.R and V.sub.B are adjusted by the manual volume 37 to increase the gain of the amplifier 22a and decrease the gain of the amplifier 22c.
In this case, it is apparent that as the gain of the amplifier 22a is increased, the level of the R signal E.sub.R will rise, and that as the gain of the amplifier 22c is decreased, the level of the B signal E.sub.B will lower. In accordance with the obtained color signals, the luminance signal E.sub.Y and color difference signals E.sub.R-Y and E.sub.B-Y are generated and encoded. As a result, an image more reddish than that before the gain change is displayed on the screen of a monitor (not shown).
(B) Next, the case will be described wherein in the negative mode, the white balance control voltages V.sub.R and V.sub.B are adjusted by the manual volume 37 to increase the gain of the amplifier 22a and decrease the gain of the amplifier 22c.
In this case, it is apparent that as the gain of the amplifier 22a is increased, the level of the R signal E.sub.R will rise, and that as the gain of the amplifier 22c is decreased, the level of the B signal E.sub.B will lower. In accordance with the obtained color signals, the luminance signal E.sub.Y and color difference signals E.sub.R-Y and E.sub.B-Y are generated, inverted and encoded. As a result, an image more bluish than that before the gain change is displayed on the screen of a monitor (not shown).
Next, the operation of the white balance control in the above case (A) will be described in more detail. For the purpose of simplicity, it is assumed that luminance and color difference matrices in conformity with the NTSC system are ideal.
The conversion of color signals E.sub.R, E.sub.G, and E.sub.B into color signals E.sub.Y, E.sub.R-Y, and E.sub.B-Y is carried out using the following equations. ##EQU2##
The inverted image signals are given by the following equations. ##EQU3##
The reproduced signals at the monitor are given by the following equations. ##EQU4## where E.sub.Rm is a red channel signal at the monitor, E.sub.Gm is a green channel signal at the monitor, and E.sub.Bm is a blue channel signal at the monitor.
From the equations (IV) and (V), the reproduced signals at the monitor during the negative mode are given by the following equations. ##EQU5##
Accordingly, as the R signal E.sub.R level is raised at the amplifier 22a, the signal E.sub.Rm at the monitor becomes weaker, and as the B signal E.sub.B level is lowered at the amplifier 22c, the signal E.sub.Bm at the monitor becomes stronger. In other words, an image on the monitor becomes bluish. This is because a reversal function of image signals is provided at the succeeding stage of the white balance controlling variable gain amplifiers.
A conventional color television camera having a negative-positive reversal function and a white balance control function is constructed as described above. Therefore, the operation of the white balance control is reversed between the negative and positive modes. In adjusting a white balance slightly by manipulating the manual volume 37, the control operation is reversed between the negative and positive modes, resulting in poor handling in operation. A color television camera, particularly a camera having a single manual volume for adjusting an image to become reddish or bluish, the rotation direction is reversed, resulting in disability of adjustment in some cases.